High performance thermoplastic elastomers are elastomers which, when formed into a film, strand or similar article, can be extended to a stretched length and retracted without experiencing a substantial loss in retractive force at an intermediate (lower) stretched length. While these elastomers typically have useful properties of high strength, low hysteresis, low creep and low stress relaxation, they are expensive relative to other elastomers and thermoplastic polymers in general.
Low performance thermoplastic elastomers are elastomers which, when formed into a film, strand or similar article, can be extended to a stretched length and retracted, but substantially lose their retractive force at an intermediate (lower) stretched length. While these elastomers are typically less expensive than high performance elastomers, they exhibit higher levels of hysteresis, creep and stress relaxation when stretched.
High and low performance elastomers have been blended together in various attempts to create blends which retain the performance characteristics of high performance elastomers to the extent possible, while saving on material costs. The blending has not resulted in elastomer blends having suitable hysteresis, creep, stress relaxation, or other properties. One reason for this is that the Gibb's free energy of mixing is positive, due to high interfacial tension between the molecules of high performance elastomer and low performance elastomer. The Gibb's free energy of mixing, or ΔG, is defined as the Gibb's free energy of the elastomer blend minus the sum of the Gibb's free energies of the components prior to blending, at a reference temperature.
When the Gibb's free energy of mixing is positive, thermodynamics favor phase separation over intimate mixing of the two elastic polymer components. Often, one of the components forms a continuous phase, while the other component forms a discontinuous phase of droplets or domains dispersed within the continuous phase. The phase separation adversely affects the elastic properties of the blend, and can result in melt instability during processing, and non-uniform gauge and physical appearance of the film or filament structure being produced. The tendency to phase separate diminishes as the Gibb's free energy of mixing approaches zero, and intimate mixing is favored when the Gibb's free energy of mixing is less than zero.
Accordingly, there is a need or desire for an elastic polymer composition including a high performance elastomer and a low performance elastomer, which combines the performance properties of the high performance elastomer with the cost benefits of the low performance elastomer. More specifically, there is a need or desire for an elastic polymer composition including a high performance elastomer and a low performance elastomer, which has a lower Gibb's free energy of mixing than a simple blend of the same high performance elastomer and the same low performance elastomer, in the same weight proportions, at ambient (storage) and elevated (mixing and processing) temperatures. There is also a need or desire for film and fabric layers formed using such an improved elastic polymer composition.